Protease-Activated Receptors (PARs) are a family of G-protein coupled receptors (GPCRs) comprising PAR-1, 2, 3, and 4. PARs are typically activated when enzymes (such as thrombin or trypsin) proteolytically cleave a portion of their N-terminal region. This cleavage exposes a region of the N-terminal extracellular domain (called the “tethered ligand”) which is believed to bind to residues contained within the second extracellular loop of the PAR receptors, resulting in the stabilization of an active conformation. Short synthetic peptides mimicking the tethered ligand sequence have been successfully used to activate all of the PAR receptors, except PAR-3.
PAR-2 is activated by several host and pathogen-derived serine proteases, including trypsin, mast cell tryptase, tissue kallikreins, and members of the coagulation cascade TF-FVIIa and FVa-FXa. Synthetic ligands such as SLIGKV-NH2 can selectively activate human PAR-2, although modified PAR-2 synthetic agonists such as 2-fluoryl-LIGRLO-NH2 have been reported to be more potent activators of this receptor.
PAR-2 has been shown to be an important receptor in mediating inflammation, pain and itch. For example, PAR-2 activation results in inflammatory cytokine and chemokine release from keratinocytes, endothelial cells and from human epithelial cell lines such as A549. Moreover, the administration of PAR-2 activating proteases and synthetic agonists in vivo induce inflammatory responses. In particular, several studies have shown that intraplantar administration of PAR-2 agonists in rodents results in an edema response that is dependent in part on neuronal PAR-2 activation.
Similar studies have implicated PAR-2 as a mediator of neurogenic inflammation, nociception and in transmission of pain. This is mediated in part by the activation of PAR-2 dependent signaling pathways in dorsal root ganglia, the release of neuropeptides from C-fibers in peripheral tissues and spinal cord and the potentiation of transient receptor potential vaniloid 1 and 4 receptors in sensory neurons.
Several studies have demonstrated a role for PAR-2 activation in pruritus. Both direct activation of PAR-2 on nerve endings and indirect effects of PAR-2 on resident cells including keratinocytes are thought to contribute to itch.
Further, both in vitro and in vivo studies have demonstrated a role for PAR-2 activation in tissue remodeling. First, activation of PAR-2 can promote fibroblast and myofibroblast proliferation, and the secretion of growth factors such as CTGF and extracellular matrix components including collagen. In addition, PAR-2 activation was shown to be implicated in cellular migration and activation of this pathway has recently been shown to promote tumor growth and metastasis.
Numerous studies relying on the use of PAR-2 deficient mice, blocking PAR-2 antibodies or PAR-2 antagonists such as GB88 revealed an important role for PAR-2 activation in the pathophysiology of a variety of diseases including asthma, chronic pain, rheumatoid arthritis, periodontitis, inflammatory bowel diseases, irritable bowel syndrome, skin diseases, cancer, fibrotic diseases and neurological disease (reviewed in Yau et al, Journal of Medicinal Chemistry, July 2013). Other studies have shown that diet-induced obesity, adipose inflammation, and metabolic dysfunction correlating with PAR-2 expression are attenuated by PAR-2 antagonism.
For all of these reasons, there is a need for the development of potent and selective inhibitors of the PAR-2 signaling pathway for the treatment of inflammation and nociception (pain) caused by inflammation, cancer, or injury.